embedded-trainings-2020/embedded-workshop-book/src/uarte-implementation.md

Write the Uarte implementation

Step-by-Step Solution

1. Check Documentation. The UART protocol requires four pins, they are usually labelled:

• RXD
• TXD
• CTS
• RTS Check the documentation to find out which pins are reserved for these and what their configuration needs to be.

2. Explore the nrf-hal to find out what needs to be done.

The nrf52840-hal is a crate that exports all the 52840 flagged features from the nrf-hal-common. Let's take a look at the Uarte module.

In line 16 we see, that the nRF52840 uses the hal::pac::UARTE1 peripheral. In line 44 you find the struct Uarte<T>(T), the interface to a UARTE instance T. Besides the instance T, the instantiating method takes variables of the following types as arguments: Pins, Parity and Baudrate.

A quick search of the document reveals where to find all of them:

• Pins: Line 463
• Parity and Baudrate: Re-export on line 34

Add the following lines as import:

use hal::pac::uarte0::{
    baudrate::BAUDRATE_A as Baudrate, config::PARITY_A as Parity};
use hal::uarte;

3. Add struct Uarte that serves as a wrapper for the UARTE1 instance.

The struct has one field labelled inner, it contains the UARTE1 instance.

4. Bring up the peripheral in the fn init()

Take a closer look at the definition of the Pins struct. Import the types of the pin configuration that you don't have yet. Note that the third and fourth pin are each wrapped in an Option. Level? Create an instance of this struct in fn init() with the appropriate pins and configurations.

Create an interface to the UARTE1 instance with uarte::Uarte::new(...). The UARTE0 instance can be found in the periph variable. Set parity to INCLUDED and the baud rate to BAUD115200.

5. Board struct

Add a field for the Uarte struct in the Board struct. add the field to the instance of the Board struct in fn init().

6. Implementing the fmt::Write trait

We can't just write to the Uarte instance. A simple write would write from flash memory. This does not work because of EasyDMA. We have to write a function that implements the fmt::Write trait. This trait guarantees that the buffer is fully and successfully written on a stack allocated buffer, before it returns.

I think this is plenty for an hour.